Conflict Between Positive and Negative Responses

Understanding the opposite growth outcome of photoreceptor action in different organs poses a real challenge, but its ultimate explanation may not be as complex as it seems at first. There are precedents in other light-related responses. For example, the time of flowering is under strict photo-period control in many species, with long days being promotive in some and repressive in others. Within a single species, tobacco, it is indeed possible to find cultivars which are long day, short day and day neutral, implying that a small number of genetic differences can account for the disparity. Although we do not know about the molecular basis of such differences among tobacco cul-tivars, comparisons between Arabidopsis (a long day plant) and rice (a short day one) have been informative (Yanovsky and Kay 2003). In both species the day length is detected through the extent of overlap between the physical presence of light, determined by photoreceptor action, and the timing of the subjective night, during which the CO protein is synthesised. In both the flowering trigger is encoded by the FT gene. The difference lies in the regulation of FT by CO: in Arabidopsis coincident light and CO promote FT expression, while in rice they inhibit it (Yanovsky and Kay 2003).

This example could help conceptualise some growth paradoxes. For example, for both radicle and hypocotyl extension GAs act as growth factors. However, during germination phytochrome activity leads to active GA production, through the transcriptional regulation of the corresponding biosyn-thetic genes, while for hypocotyl extension in the light it is the inactivation of phytochrome (brought about by shade light or by mutations) that brings about the production of the hormone.

Explaining the contrasting, but simultaneous, growth response of different organs might require more complex explanations, and indeed may require organ-specific signal transduction changes. In a few cases, however, the contrasting responses may be explained by a combination of resource allocation and flow of growth regulators between organs. One good example is the reduction in root growth when shade or loss-of-phyB promote longer hypocotyls or shoots (Salisbury et al. 2007). Altered auxin flow from shoot to root has been shown to be associated with this phenomenon. The extent to which loss of phyB in cucumber leads to biomass redistribution, away from leaf blades and roots and into elongating hypocotyl and leaf petioles, is illustrated in Fig. 2c.